1. Field of the Invention
The present invention relates to a curtain coater for coating a layer of liquid coating compositions on a moving continuous web in the manufacture of a photographic element.
2. Description of the Prior Art
In coating apparatus of the curtain type, a moving web is coated by causing a free falling vertical curtain of coating liquid to impinge onto the moving web to form a layer on said web. An apparatus is described in U.S. Pat. No. 3,508,947 wherein a multilayer composite of a plurality of distinct layers is formed on a slide hopper and caused to impinge onto an object or moving web to form a coated layer thereon.
In the curtain coating process, particularly as used to manufacture multilayer photographic materials, the quality of coating is largely determined by the properties of the liquid curtain. It is important to insure that a stable laminar flow of liquid film is formed by the slide hopper and that an equally stable, laminar flow liquid curtain is formed from that film. To prevent contraction of the falling curtain under the effect of surface tension, it is known that the curtain must be guided at its edges by curtain edge guides.
The edge guides are solid members which are attached to the hopper used to supply coating liquid to the curtain and extend downwardly from the initial point of free fall of the curtain. Wetting contact of the edges of the falling curtain with the edge guides should be maintained over the entire length of the guides to avoid a contraction of the curtain.
These edge guides give rise to some problems. The wetting contact of the edges of the curtain with the guides causes non-uniformities of the coating if the full width of the curtain is applied to the moving web. By making the curtain wider than the web to be coated by an amount at least equal to this non-uniform region at each edge, a substantially uniform coating can be formed on the web. The coating liquid at the margins of the curtain that overflows the edges of the travelling web can be collected. However there is a serious risk of the coating, which is wetting those edges, to run onto the reverse margin of the web. This can cause soiling of the web transporting rollers of the web gelling and drying stations so that the production process must be interrupted to clean the rollers. Therefore, the distance between the two edge guides is preferably smaller than the width of the web.
In order to avoid the problem of the non-uniformities of the coating at the edges a lot of methods and devices are disclosed in e.g. U.S. Pat. No. 3,508,947, EP 0,176,632 and U.S. Pat. No. 4,830,887. All these techniques require the lower end of the edge guides be positioned as close as possible to, or even making contact with, the web.
It is very important for a curtain coater to uniformly apply a freely falling curtain of a liquid to a moving web at the start of the coating. If the liquid is not uniformly applied to the moving web at the start of the operation, problems such as contamination due to spattering of the applied liquid and contamination of a roller due to non-drying of greater thickness portions of the applied liquid downstream to a drying zone can occur, making the final product unacceptable. Therefore for a modern production process, it is of great importance to reduce these startlosses to a minimum.
In the curtain coater disclosed in the U.S. Pat. No. 3,508,947 an intercepting start plate, which is a turnable or slidable deflector, is provided which deflects a quantity of the liquid at the start of the coating in order to enable an operator to take measures for the formation of a uniform and stable curtain. The deflected coating liquid may be recovered. EP 0 344 745 A1 describes another embodiment of the same technique; a turnable start plate has a fulcrum under the backup roller.
When the lower end of the edge guides has to be postioned as close as possible to the web, there is simply no space left for a start plate to be positioned between edge guides and web. It is not practically possible to use a start plate that is smaller than the distance between the two edge guides, since it will not gather all the liquid and the remaining will soil inevitably the web in a non-uniform way. A solution known in the art is to move the entire coating device together with the edge guides and interceptors or other means to avoid coating thick edges, as described in the patents mentioned before, to an upward position, in order to allow a start plate to be positioned underneath. When a uniform and stable liquid curtain has been established, the start plate can be removed and the coating device is lowered in operation position. This operation is mechanically difficult to execute and time consuming. During this operation the curtain will contract due to the fact that the edge guides are not close enough to the web. The coated web produced during this operation is therefore not suited for the desired application.